In injection molding, a molten thermal plastic or similar material is injected into a mold having a closed cavity having outer surfaces conforming to the shape of the desired part. After injection, the mold may be separated along a parting line to release the molded part and then closed again so that the injection process may be repeated.
In multi-shot injection molding, a first shot of injected plastic is delivered to the mold and then a portion of the mold cavity is changed and a second shot of plastic is injected into the changed cavity. Multi-shot injection molding may be used to create parts having two or more different thermoplastic materials that are joined as an integral unit or that are separable after molding is complete. In this latter case, the molding process effectively pre-assembles the separate plastic components in the mold.
The reconfiguration of the mold, in between the separate injections of plastic (shots), is easily performed by rotating a portion of the mold. The rotation may bring successive portions of the mold into alignment with different injector units so that the multiple shots may be accomplished simultaneously on different portions of the mold, thereby improving machine throughput. A well known technique for mold rotation involves rotating a center portion of a three part mold along an axis perpendicular to the direction in which the mold portions separate (the mold separation direction).
During a first stage, a first material may be injected into a front cavity at the interface of a front and center portion of the mold to create a base part. At the same time, a second material may be injected into a rear cavity formed by the interface of the center and rear portions of the cavity. This rear cavity contains a base part previously molded in the front cavity and the second material over molds the part.
In a second stage, the mold portions are separated and the over molded part is ejected from the rear cavity and the mold rotated 180xc2x0. The molds portions are closed again and the process is repeated.
Rotation of the center portion of the mold about an axis perpendicular to the direction of mold separation reduces the required clamping pressure on the molds and may be contrasted to a system that rotates the molds about an axis parallel to the mold separation direction. This latter system increases the total area of the molds over which the injected plastic presses, requiring additional clamping pressure and possibly a larger injection-molding machine.
While rotating the center mold portion perpendicularly to the mold separation direction requires less total clamping pressure, greater separation of the machine platens is required to provide for mold rotation and part removal. The center mold portion is supported during separation by the tie rods, which join the platens of the injection-molding machine. The location and size of these tie rods varies between machines and thus such an approach requires that the molds be designed for a specific machine.
A variation on this approach holds the rotatable mold portion within a frame held by cantilevered lead pins extending from the front and rear mold portions and received by this frame. This approach is limited to relatively small molded parts using lightweight molds that separate only the short distance supportable by the cantilevered pins. One drawback is that the frame limits the area of the platens that may be used for the mold and thus the size of the molded part.
What is needed is a mold support system adaptable to arbitrarily large molds that does not require the use of a specific injection-molding machine.
The present invention provides a mold support for a rotating central mold portion that is supported solely through the platen mold mounts and thus does not require extensive modification of an injection-molding machine or the use of an injection-molding machine having its tie rods in a particular location. The invention provides a frameless design in which the mold is supported from beneath on a rotating table held by rails attached to the platens. Positioning of the rails below the mold allows the rail to present a support span in the mold separation direction substantially wider than the mold itself allowing improved stability, greater mold sizes, and greater clearance when the molds are separated.
Specifically, the present invention provides a rotating mold support having a first and second platen mount attachable to the opposed platens of a standard injection molding machine and providing opposed mold attachment surfaces for first and second mold portions, respectively, and adjacent track attachment surfaces. At least one extensible support track is attached to extend between the track support areas of the first and second platen mount to span the distance between the first and second platen mount for a plurality of separations of the first and second platen mounts along a mold separation axis. A rotatable mold stage is attached to a middle part of the extensible support track and has an upper surface supporting a third mold portion for rotation about an axis perpendicular to the mold separation axis.
It is thus one aspect of the invention that it allows the frameless support of the center portion of the mold improving the capacity of an injection-molding machine to mold large parts.
It is another aspect of the invention that it allows the mold to be supported on a wide variety of different injection molding machines without extensive modification to those machines. Because the rotatable mold stage is supported by tracks held by the platen mounts, only platen mounting is required to attach the mold to a given machine.
The track attachment surfaces may be below the mold attachment surfaces so that the track is below the third mold portion. The third mold portion may be supported solely by the upper table surface during rotation.
Thus it is another aspect of the invention that it provides an extremely simple mechanism for supporting a mold portion for rotation such that allows ready access to the third mold portion.
The extensible support track may include a first and second parallel rail extending along the mold separation axis. The first parallel rail may be attached to the first platen mount and the second parallel rail may be attached to the second platen mount, and the first and second parallel rails may slidably engage with a support element over a support span, the support element forming the middle part of the extensible support track.
Thus it is another aspect of the invention that the support track may provide for telescoping action increasing its usable range.
The support span may have a width measured along the mold separation axis greater than the width of the third mold portion measured along the mold separation axis.
Thus it is another aspect of the invention that it may provide an extremely sturdy extensible support track that can provide support points much exceeding the size of the mold for improved stability against torsion and the like.
The parallel rails may have a cross-sectional height measured vertically that is more than twice their cross-sectional width measured horizontally.
It is a feature of some embodiments of the invention to allow advantageous rail cross-sections by displacing the extensible track away from the mold area.
The first and second parallel rails may include opposed racks and the support element may be positioned between the first and second parallel rails and include a gear simultaneously engaging both racks to cause the support element to move equally with respect to the first and second platen supports.
It is thus another feature of at least one embodiment of the invention that the third mold portion may be positioned to remain centered between the first and second mold portions as the latter open.
The first and second parallel rails may include opposed ways and the support element may be positioned between the first and second parallel rails and may include slides engaging the opposed ways of the first and second parallel rails to move freely along each.
It is another aspect of the invention that it provides for an improved slider mechanism over that obtained by leader pins alone.
The features and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.